1. Field of the Invention
The present invention relates to a transmitter/receiver for radio communication using an amplitude demodulation method, an RFID system which uses this transmitter/receiver for radio communication as a RFID reader/writer, and a receiving method for this transmitter/receiver for radio communication.
2. Related Background Art
In an RFID system, information stored in an RFID tag can be read or information can be written to an RFID tag, using an RFID reader/writer (transmitter/receiver for radio communication). For example, an RFID reader/writer sends a command signal to an RFID tag, and the RFID tag replies with a response signal to this command signal. Receiving this response signal, the RFID reader/writer performs a processing corresponding to the response signal (e.g. above mentioned reading information or writing information).
In an RFID tag, a load modulation system is widely used as a response signal reply system. According to the load modulation system, an RFID reader/writer sends an unmodulated transmission signal via an antenna, and an RFID tag side switches load impedance connected to the antenna in the RFID tag so as to change the conditions of reflection from the antenna of the RFID tag, and in result, an amplitude change, generated in an antenna circuit of the RFID reader/writer, is detected. Therefore in the RFID reader/writer, an amplitude demodulation system is used to demodulate the response signal from the RFID tag.
This type of RFID reader/writer is explained in the “RFID Handbook”, Second Edition, written by Klaus Finkenzeller, translated by Software Engineering Research Center, Nihon Kogyo Shimbum Co., May 31, 2004, p. 287, FIG. 11, FIG. 16. This RFID reader/writer, which performs amplitude demodulation using a diode, demodulates a superimposed signal of the transmission signal of the reader/writer and the receive signal from the RFID tag.
However, using the RFID reader/writer described in the “RFID Handbook”, Second Edition, written by Klaus Finkenzeller, translated by the Software Engineering Research Center, Nihon Kogyo Shimbum Co., May 31, 2004, p. 287, FIG. 11 and FIG. 16, a communication null phenomena may occur under specific conditions. For example, depending on a specific distance condition between the RFID reader/writer and the RFID tag, the RFID reader/writer may not be able to demodulate the response signal from the RFID tag, even if this response signal has sufficient intensity.